Modern airports incorporate specialized lighting systems including, for instance, runway touchdown zone lighting systems, runway centerline lighting systems, taxiway centerline lighting systems, and edge lighting systems. Edge lighting systems are important because they help to define runways and taxiways and inhibit pilots from inadvertently driving their airplanes off the designated runway and taxiway areas.
Two of the most important types of lighting fixtures of airport lighting systems are guidance signs, and runway guard lights. Guidance signs are used on airport taxiways to mark taxi routes and intersections. Guidance signs are provided in different colors, including black-on-yellow, yellow-on-black, or white-on-red. The color of a guidance sign depends on the application of the guidance sign as defined by applicable governing standards. A runway guard light is an elevated or in-pavement flashing yellow light fixture that provides distinctive warnings to pilots that they are approaching a runway holding position and are about to enter an active runway.
Guidance signs operate at night, and during periods of poor visibility, such as during bad weather and during foggy conditions. During daylight hours when visibility is not impaired due to poor weather conditions, the internal lighting of guidance signs is turned OFF in order to save on electrical and maintenance costs. Moreover, governing standards do not require guidance signs to be operational during daylight hours when visibility is not impaired due to poor weather conditions. However, runway guard lights must be operational at all times as required by applicable governing standards.
Guidance signs of airport lighting systems are wired electrically in series with a guidance sign circuit, which may include a dedicated guidance sign circuit, a taxiway edge circuit, or a runway edge light circuit. In order for the runway guard light to operate in times of good visibility when the runway edge or taxiway edge circuits are turned OFF, runway guard lights of airport lighting systems are also wired electrically in series with a runway guard light circuit. The electrical wiring of a guidance sign circuit providing electrical power to guidance signs is separate from the electrical wiring of the runway guard light circuit providing electrical power to the runway guard lights. This is important and currently required due to the different operational characteristics of guidance signs and runway guard lights. In particular, runway guard lights must not be turned off. Guidance signs can be turned OFF manually, such as by the air traffic controllers in the air traffic control center or tower, or automatically, such as by a timer or photocell. If runway guard lights were connected to the same electrical infrastructure as the guidance signs, the runway guard lights could accidentally be turned OFF due to operator error or a systematic error, which could lead to disastrous results.
Although it is important and currently necessary for guidance signs and runway guard lights to have their own electrical wiring infrastructure, wiring an airport with separate guidance sign and guard light electrical wiring infrastructures is very expensive, difficult, and time consuming. Also, the cost of the electricity required to constantly power runway guard lights is also very expensive.